1. Field of the Invention
The present invention relates to an image forming apparatus used in a copying machine or a laser printer. More specifically, it relates to an image forming apparatus in which the generation of ozone by corona charging can be prevented effectively and a photosensitive drum is not scratched and the photosensitive drum has a long service life.
2. Description of the Prior Art
In electrophotographic copying machines, a high voltage is supplied by corona discharge. When a corona discharge is generated, oxygen in air is oxidized to generate ozone. The ozone gives rise to a problem of environmental pollution, or a problem of deteriorating the photosensitive material.
Many photosensitive materials now in practical use are negatively charged. In the negatively charged organic photosensitive materials, the amount of ozone generated for utilizing a negatively corona discharging is especially large.
Various proposal have previously been made in order to prevent the generation of ozone by corona discharge in such an image forming apparatus. For example, it is known to provide a method which comprises contacting a soft fibrous contacting piece with a material to be charged, thereby to electrify the material (Japanese Laid-Open Patent Publication No. 269975/1987).
However, the method of electrification by contacting a material to be charged may charge a photosensitive material drum without using corona discharge and therefore prevent the generation of ozone. But since the contacting piece carries out electrification by scratching the surface of the photosensitive drum which is a material to be charged, it is likely that a scratch may be formed on the surface of the drum. Furthermore, because a high electric field is formed between the forward end of the contacting piece and the photosensitive drum, the current leaks to form pinholes in the photosensitive drum and leads to defects such as the short service life of the photosensitive material drum.
Furthermore, it is difficult to contact the contacting piece uniformly with the surface of the photosensitive material drum. Although it may be possible to apply a high voltage to a portion directly contacted, it is difficult to fully electrify a portion which the contacting piece does not directly contact. This leads to a problem that electrification unevenness occurs in the surface of the photosensitive material drum.
Furthermore, when cleaning is insufficient in the photosensitive material drum, the toner or the paper powder remains on the photosensitive material drum. When a charging treatment is carried out by the above method on the photosensitive material drum which is insufficiently cleaned, the toner, etc. remaining on the drum contaminates the contacting piece, and electrification unevenness will occur. Such uneven electrification adversely affects the formation of an image, and unpreferably cannot obtain a good time.